Diamond material

ABSTRACT

The present disclosure relates to a method of making fancy orange synthetic CVD diamond material. Among other things, the method may involve (i) providing a single crystal diamond material that has been grown by CVD and has a [N s   0 ] concentration less than 5 ppm; (ii) irradiating the provided CVD diamond material so as to introduce isolated vacancies V into at least part of the provided CVD diamond material such that the total concentration of isolated vacancies [V T ] in the irradiated diamond material is at least the greater of (a) 0.5 ppm and (b) 50% higher than the [N s   0 ] concentration in ppm in the provided diamond material; and (iii) annealing the irradiated diamond material to forming vacancy chains from at least some of the introduced isolated vacancies.

BACKGROUND OF THE INVENTION

This invention relates in general to a method of making a fancy orangesingle crystal diamond material by post-growth treatment of a diamondmaterial that has been grown by a CVD (chemical vapor deposition)process, and to CVD single crystal diamond material which is fancyorange in color.

The term “fancy-colored diamond” is a well-established gem tradeclassification and is used to refer to unusual colored diamonds. Auseful history and background to the grading of fancy colored diamondgemstones, including the use of Munsell color charts is given by King etal, in Gems & Gemology, Vol. 30, No. 4, 1994 (pp. 220-242).

Examples of fancy colored synthetic and natural diamonds made byintroducing color centers into the diamond are known in the prior art.For example, EP0615954A and EP0316856A describe irradiation of syntheticdiamond material with an electron beam or a neutron beam to form latticedefects (interstitials and isolated vacancies) in the crystal.Thereafter the diamond crystal is annealed in a prescribed temperaturerange to form color centers. Neither of these publications disclosesorange diamond material.

Another publication describing the formation of fancy colored diamondmaterial is “Optical Absorption and Luminescence” by John Walker in“Reports on Progress in Physics”, Volume 42, 1979. That publicationsimilarly describes the steps of forming lattice defects in crystals byelectron beam irradiation, and if necessary annealing to cause thelattice defects to combine with nitrogen atoms contained in thecrystals. There is no disclosure of orange diamond material in thispublication.

US 2004/0175499 (Twitchen et al) describes a method starting with acolored CVD diamond, usually brown or near-brown, and applying aprescribed heat treatment to produce another and desirable color in thediamond. The prior art reference notes that the relative strengths ofthe absorption bands in the visible region of the spectrum of brownsingle crystal CVD diamond can be altered by annealing, with concurrentchanges in the Raman spectrum, and that changes in the absorptionspectrum are observed at much lower temperatures than are required toalter the color of brown natural diamond. Significant color changes aresaid to be achieved by annealing at atmospheric pressure in an inertatmosphere at temperatures of 1600° C. or less. One example describes agrown CVD diamond polished into a round brilliant of 0.51 carat that wasgraded as light brown. After annealing at 1700° C. for 24 hours it wasgraded as light orangish pink. Another example describes a grown CVDdiamond slice which had an orange brown color, and after annealing thiscolor becomes colorless. A further example describes a grown CVD diamondlayer polished into a rectangular cut gemstone of 1.04 carats which isgraded fancy dark orangey brown color. After annealing at 1600° C. forfour hours this becomes a fancy intense brownish pink color.

SUMMARY OF THE INVENTION

Embodiments of a method for making diamond material with an orangecolor, and orange diamond material per se are disclosed. Amongst theseembodiments, we have found that a fancy orange color can be introducedinto synthetic CVD diamond material by irradiating synthetic CVD diamondmaterial for a time sufficient to introduce a specified concentration ofisolated vacancies into the diamond material, and then annealing thatisolated-vacancy-containing CVD diamond material for a sufficiently longtime at a low temperature to produce a fancy orange colored diamondmaterial. Without limiting the invention in any way it is thought thatduring the low temperature anneal at least some of those isolatedvacancies are converted into vacancy chains in the CVD diamond materialand that the vacancy chains are responsible for the perceived fancyorange color of the diamond material.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the invention provides a method of making fancy orangesynthetic CVD diamond material, the method comprising: (i) providing asingle crystal diamond material that has been grown by a CVD process andhas a [N_(s) ⁰] concentration less than 5 ppm; (ii) irradiating theprovided CVD diamond material so as to introduce isolated vacancies Vinto at least part of the provided CVD diamond material such that thetotal concentration of isolated vacancies [V_(T)]=([V⁰]+[V⁻]) in theirradiated diamond material is at least the greater of (a) 0.5 ppm and(b) 50% higher than the [N_(s) ⁰] concentration in ppm in the providedCVD diamond material, and (iii) annealing the irradiated diamondmaterial at a temperature of at least 700° C. and at most 900° C. for aperiod of at least 2 hours, optionally at least 4 hours, optionally atleast 8 hours thereby forming vacancy chains from at least some of theintroduced isolated vacancies.

According to the first aspect of the invention, the total concentrationof isolated vacancies [V_(T)]=([V⁰]+[V⁻]) in the irradiated diamondmaterial is at least the greater of (a) 0.5 ppm and (b) 50% higher thanthe [N_(s) ⁰] concentration in ppm in the provided CVD diamond material.This means that the total concentration of isolated vacancies[V_(T)]=([V⁰]+[V⁻]) always has a minimum value of 0.5 ppm even for lowor zero [N_(s) ⁰] concentrations in the provided CVD diamond material.Above [N_(s) ⁰] concentrations in the provided CVD diamond material ofabout 0.33 ppm the minimum value of the concentration of isolatedvacancies [V_(T)]=([V⁰]+[V⁻]) in the irradiated diamond material isgiven by calculating 50% higher than the [N_(s) ⁰] concentration in ppmin the provided CVD diamond material, since that will result in a valuefor the concentration of isolated vacancies [V_(T)]=([V⁰]+[V⁻]) greaterthan 0.5 ppm.

In some embodiments according to the invention the irradiation andannealing steps are carried out so as to reduce the concentration ofisolated vacancies, [V_(T)] to a concentration of <0.3 ppm.

Fancy orange colored diamond material made by the method according tothe present invention may be used as gemstones. Other applications, forexample use as a color filter or cutting tool for example a scalpel arealso envisaged.

The CVD diamond prior to the irradiation step is referred to sometimesin this specification as “the provided CVD diamond”. The step ofactually growing the CVD diamond material may or may not form part ofthe method of embodiments of the invention. Providing a CVD diamondmaterial may simply mean, for example, selecting a pre-grown CVD diamondmaterial. The diamond material after irradiation step (i) is referred toas the “irradiated diamond”, and the diamond material after theirradiation and annealing step is referred to as the “treated diamondmaterial” or as the “irradiated and annealed diamond material”. Steps(i) to (ii) of embodiments of methods of the present invention,describing the diamond material at each stage of the method areillustrated in the flow diagram of FIG. 1.

The provided CVD diamond material in the method of the present inventionhas a [N_(s) ⁰] concentration (that is a concentration of neutral singlesubstitutional nitrogen defects) less than 5 ppm, optionally less than 4ppm, optionally less than 3 ppm, optically less than 2 ppm, optionallyless than 1 ppm. The color of the provided CVD diamond material may varyaccording to the [N_(s) ⁰] concentration, and the manner in which thediamond material has been grown. It is known that [N_(s) ⁰] defectsthemselves introduce a yellow coloration into diamond material,particularly at concentrations greater than 0.3 ppm, but the skilledperson will recognize that the observation of color is related to boththe concentration and the optical path length through the diamond. It isalso known that the presence of the low concentrations of nitrogen in aCVD growth environment can affect the nature and concentration of otherdefects that are incorporated in a CVD synthetic diamond material as thediamond material grows, and that at least some of these other defectsprovide color centers contribute to the color of CVD diamond material,typically introducing a brown coloration to the diamond material. Allmeasurements to calculate the concentration of N_(S) ⁰ are donefollowing UV excitation.

It is thought that the color centers that contribute to the browncoloration of CVD diamond grown in the presence of low concentrations ofnitrogen are unique to single crystal CVD diamond, or to pieces cut orproduced from layers of single crystal CVD diamond. It is furthermoreknown that the color centers contributing to brown coloration in CVDdiamond are different from those contributing to any brown colorationobserved in natural diamond because the defects in the CVD diamondmaterial cause absorption bands in the absorption spectra of the grownCVD diamond material that are not found in the absorption spectra ofnatural diamond. Evidence for this comes from Raman scattering fromnon-diamond carbon observable with an infrared excitation source (e.g.785 nm or 1064 nm) which is not observed for brown natural diamond.Further, it is known that these color centers in natural diamondmaterial anneal at a different temperature to those in CVD diamondmaterial.

It is believed that some of the color centers contributing to the browncoloration seen in CVD synthetic diamond grown in processes in which lowconcentrations of nitrogen are introduced relate to localized disruptionof the diamond bonding within the single crystal CVD diamond. The exactnature of the defects is not fully understood, but the use of electronparamagnetic resonance (EPR) and optical absorption spectroscopytechniques have been used to study the nature of the defects and improveour understanding somewhat. The presence of the nitrogen in the grownCVD synthetic diamond material can be evidenced by looking at absorptionspectra for the grown CVD diamond material, and analysis of thesespectra gives some indication of the relative proportions of differenttypes of defect present. A typical spectrum for grown CVD syntheticdiamond material grown with nitrogen added to the synthesis environmentshows a peak at about 270 nm, which is generated by the presence ofneutral single substitutional nitrogen (N_(s) ⁰) atoms in the diamondlattice. Additionally peaks have been observed at about 350 nm andapproximately 510 nm corresponding to other defects characteristic andunique to CVD synthetic diamond material, and furthermore a so-called“ramp”, that is a rising background of the form c×λ⁻³ has been observed,where c is a constant and λ is the wavelength. While the N_(s) ⁰ isprimarily identifiable by its peak at 270 nm, it also contributes insmaller amounts to the absorption spectrum at higher wavelengths, inparticular at wavelengths in the visible part of the spectrum, which isgenerally considered to cover the wavelength range 350 nm to 750 nm.

It is the combination of features evident in the visible part of theabsorption spectrum of the CVD diamond material, i.e. (a) the N_(s) ⁰contribution in the visible part of the spectrum, (b) the 350 nm peak,(c) the 510 nm peak and (d) the ramp feature, that affect the perceivedcolor of the diamond material and are believed to be responsible for thebrown color typically seen in nitrogen doped CVD synthetic diamondmaterial. The peaks at 350 nm and 510 nm, are not seen in the absorptionspectra of natural diamonds, nor in the absorption spectra of othersynthetic diamonds, for example synthetic HPHT diamonds of the typedescribed in EP615954A. For the purposes of this specification, alldefects other than the N_(s) ⁰ defects that contribute to the absorptionspectrum in the visible part of the spectrum, which we have discussedabove as the 350 nm, 510 nm and ramp features, will be referred tocollectively as “X defects”. As noted above, at this time the structuralnature of these defects at an atomic level is not understood, merelytheir effect on the grown diamond material's absorption spectra. Withoutbinding the invention in any way, it is thought that the nature of thedefects responsible for the brown coloration might be related to thepresence of multi-vacancy clusters (each cluster being made up of tensof vacancies e.g. 30 or 40 vacancies or more) that are grown-in underlarge growth rates, concomitant with the addition of nitrogen to theplasma to a hydrogen/methane (H₂/CH₄) source gas. Such clusters arethermally unstable and may be removed to some degree, byhigh-temperature treatment (i.e. annealing). It is thought that smallervacancy-related defects, such as a NVH (nitrogen-vacancy-hydrogen)defects that are made up of nitrogen and hydrogen and a missing carbonatom, may be partially responsible for the brown color and these defectsmay also be removed by high-temperature treatment.

In certain methods according to the invention, the absorptioncoefficients at 350 nm and 510 nm for the provided diamond material areless than 3 cm⁻¹ and 1 cm⁻¹ respectively, optionally less than 2 cm⁻¹and 0.8 cm⁻¹ respectively.

Depending on the method of manufacture, and the [N_(s) ⁰] concentrationin the as-grown CVD diamond material, the provided CVD diamond materialused in methods according to the invention may typically appearcolorless, near colorless, or yellow or brown with weak to mediumsaturation C* and very light to medium lightness L* (C* and L* arediscussed in detail later in this specification). The [N_(s) ⁰]concentration in the provided diamond material is less than 5 ppm, whichlimits any yellow coloration of the diamond material. For certainembodiments according to the invention the absorption coefficient at 350nm and 510 nm are less than 3 cm⁻¹ and 1 cm⁻¹ respectively, optionallyless than 2 cm⁻¹ and 0.8 cm⁻¹ respectively, the absorption coefficientsat these wavelengths being a measure of the brownness of the diamondmaterial since the X defects are thought to be responsible for much ofthe brown coloration due to above-mentioned X defects in diamondmaterial grown by a CVD process incorporating nitrogen in the sourcegas.

According to different embodiments of methods according to the presentinvention, the provided CVD diamond may or may not contain N_(s) ⁰.Where it does contain N_(s) ⁰, the concentration of [N_(s) ⁰] present inthe synthetic CVD diamond material of the present invention may bemeasured using EPR for levels <5×10¹⁵ cm⁻³ and using UV visible opticalabsorption techniques for higher concentrations. These techniques areapplied to samples post exposure to UV light.

[N_(s) ⁰] in the neutral charge state content can be measured by usingelectron paramagnetic resonance (EPR). Whilst the method is well-knownin the art, for completeness it is summarized here. In measurementsconducted using EPR, the abundance of a particular paramagnetic defect(e.g. the neutral single-substitutional nitrogen defect) is proportionalto the integrated intensity of all the EPR absorption resonance linesoriginating from that centre. This permits the concentration of thedefect to be determined by comparing the integrated intensity to thatwhich is observed from a reference sample, provided care is taken toprevent or correct for the effects of microwave power saturation. Sincecontinuous wave EPR spectra are recorded using field modulation, doubleintegration is required to determine the EPR intensity and hence thedefect concentration. To minimize the errors associated with doubleintegration, base line correction, finite limits of integration, etc.,especially in cases where overlapping EPR spectra are present, aspectral fitting method (using a Nelder-Mead simplex algorithm (J. A.Nelder and R. Mead, The Computer Journal, 7 (1965), 308)) is employed todetermine the integrated intensity of the EPR centers present in theexample of interest. This entails fitting the experimental spectra withsimulated spectra of the defects present in the example and determiningthe integrated intensity of each from the simulation. Experimentally itis observed that neither a Lorentzian nor Gaussian line shape provides agood fit to the experimental EPR spectra, therefore a Tsallis functionis used to produce the simulated spectra (D. F. Howarth, J. A. Weil, Z.Zimpel, J. Magn. Res., 161 (2003), 215). Furthermore, in the case of lownitrogen concentrations, it is often necessary to use modulationamplitudes approaching or exceeding the line width of the EPR signals toachieve a good signal/noise ratio (enabling accurate concentrationdetermination within a reasonable time frame). Hence pseudo-modulationis employed, with the Tsallis line shape in order to produce a good fitto the recorded EPR spectra (J. S. Hyde, M. Pasenkiewicz-Gierula, A.Jesmanowicz, W. E. Antholine, Appl. Magn. Reson., 1 (1990), 483). Usingthis method the concentration in ppm can be determined with areproducibility of better than ±5%.

The technique of UV-visible absorption spectroscopy for measuring higher[N_(s) ⁰] concentrations is well-known in the art, and involvesmeasurements using the 270 nm peak of the absorption spectrum of thediamond material.

The provided diamond material according to the present invention may begrown using a conventional CVD process, for example of the typedisclosed in WO 03/052177. Such a process, as noted above may result ina diamond material having some brown coloration, but providing thisbrown coloration is not too strong, it can be masked by the introducedorange coloration resulting from the post growth irradiation andannealing treatment of the method of the present invention.

Another CVD growth process that may be used to produce the provided CVDdiamond material is a CVD growth process in which the source gascontains carbon, hydrogen, nitrogen and oxygen, rather than the moreusual carbon hydrogen and nitrogen. For example, oxygen may be added tothe process gas at a concentration of at least 10000 ppm in the gasphase. In particular, the provided CVD diamond material in step (i) ofthe method according to the first aspect of the invention may be growndirectly by the process described in GB application number GB0922449.4and US provisional application number U.S. Ser. No. 61/289,282 theentire disclosures of which are incorporated herein by reference.Specifically the method involves providing a substrate; providing asource gas; and allowing homoepitaxial diamond synthesis on thesubstrate; wherein the synthesis environment comprises nitrogen at anatomic concentration of from about 0.4 ppm to about 50 ppm; and whereinthe source gas comprises: (a) an atomic fraction of hydrogen H_(f), fromabout 0.4 to about 0.75; (b) an atomic fraction of carbon, C_(f), fromabout 0.15 to about 0.3; (c) an atomic fraction of oxygen, O_(f), fromabout 0.13 to about 0.4; wherein H_(f)+C_(f)+O_(f)=1; wherein the ratioof atomic fraction of carbon to the atomic fraction of oxygenC_(f):O_(f), satisfies the ratio of about 0.45:1<C_(f):O_(f)< about1.25:1; wherein the source gas comprises hydrogen atoms added ashydrogen molecules, H₂, at an atomic fraction of the total number ofhydrogen, oxygen and carbon atoms present of between 0.05 and 0.4; andwherein the atomic fractions H_(f), C_(f) and O_(f) are fractions of thetotal number of hydrogen, oxygen and carbon atoms present in the sourcegas. This method of growing CVD diamond material shall be referred to inthe specification as the “added oxygen CVD growth process”. It typicallyresults (depending on the nitrogen concentration) in a provided CVDdiamond material which is colorless, near colorless or has low browncoloration.

The color of the irradiated diamond material is a combination of thestarting color, if any, of the provided diamond material and the orangecolor introduced by the irradiation and annealing steps to introducevacancy chains. Other impurities that could introduce color into theprovided diamond material may in certain embodiments be minimized. Forexample, uncompensated boron (isolated boron) may itself introduce ablue color to the diamond material. For some embodiments the atomicboron concentration [B] in the provided diamond material is less than5×10¹⁵ cm⁻³.

It is known that if there is uncompensated boron in a diamond materialthis may be compensated for by irradiating to introduce isolatedvacancies, the isolated vacancies combining with the boron so thatneither the boron, nor those compensating isolated vacancies impart anycolor to the diamond material. Therefore in some embodiments accordingto the present invention, if the diamond material does containuncompensated boron (for example in a concentration of >5×10¹⁵ cm⁻³),then the irradiation step may be carried out so as to introducesufficient isolated vacancies not only to compensate the boron but alsoto achieve the specified isolated vacancy concentration [V_(T)]. Thelevel of additional irradiation needed for boron-compensation could bedetermined empirically by the person skilled in the art. Thus in someembodiments of method according to the invention, uncompensated boron ispresent in the provided diamond material in a concentration of >5×10¹⁵cm⁻³, and the irradiation step introduces sufficient isolated vacanciesinto the diamond material so that total concentration of isolatedvacancies [V_(T)] in the irradiated diamond material, after isolatedvacancies have been used to compensate the boron, is at least thegreater of 0.5 ppm or 50% higher than the [N_(s) ⁰] concentration in ppmin the provided CVD diamond material. The level of additionalirradiation needed for boron compensation could be determinedempirically by the person skilled in the art. Total boron in thematerial may be quantified using techniques known to the skilled man.Secondary ion mass spectroscopy (SIMS) may be used for example toascertain the total boron concentration. The uncompensated boron may beascertained using either the induced absorption measured in the infraredpart of the diamond spectrum, or through Hall or electrical transportmeasurements, in a manner known to the skilled person.

Typically the [N_(s) ⁰] concentration in ppm in the provided CVD diamondmaterial will remain substantially unchanged by the irradiation step(step (ii) of methods according to the invention). It will be changed bythe annealing step (step (iii) of methods according to the invention, asexplained later in this specification.

The provided CVD diamond material used in the method according to thefirst aspect of the present invention optionally may have at least about50%, alternatively at least about 80%, alternatively at least about 90%,alternatively at least about 95% of the volume of the synthetic CVDdiamond material formed from a single growth sector. This single growthsector is optionally a {100} or a {110} growth sector. The material ofthe single growth sector optionally has N_(s) ⁰ levels within ±10% ofthe mean for greater than about 50% of the volume of the growth sector,alternatively greater than about 60% of the volume of the growth sector,alternatively greater than about 80% of the volume of the growth sector.Using a provided synthetic CVD diamond material that has been grown froma single growth sector is advantageous as the CVD diamond material willhave fewer surfaces with different crystallographic orientation (whichare the surfaces corresponding to differing growth sectors). Surfaceswith different crystallographic orientations exhibit stronglydifferential uptake of nitrogen impurity and a synthetic CVD diamondmaterial comprising more growth sectors therefore tends to show moreundesirable zones with different color, resulting from the differentconcentrations of N_(s) ⁰ in different growth sectors.

The color of a diamond material colored by using a post growth treatmentmethod is the color of the diamond material prior to post growthtreatment combined with the effect on color of any defect producedduring the post growth treatment. According to the method of the presentinvention, we have found that if we apply a particular post-CVD-growthtreatment we can introduce an orange color in to the diamond material.Small to moderate amounts of yellow or brown in the starting materialmay be tolerated and the treated diamond (post irradiation and annealaccording to the invention) will still appear orange, since the orangecoloration introduced by the post growth treatment has moderate tostrong saturation (C* as hereinafter described), and medium to lightlightness (L* as hereinafter described) and is therefore able to masksmall to moderate amounts of yellow or brown in the provided CVDdiamond. For certain embodiments according to the invention we startwith a provided CVD diamond that has minimal or no color, i.e. issubstantially colorless; for other embodiments according to theinvention we start with a provided diamond material that has some color,usually some yellow or brownness. For example, for some embodiments, toproduce a light orange diamond material, which has a low C* and/or highL* value, (e.g. C*<10, and/or L*>65) it would be necessary to start witha colorless or pale yellow material.

According to the method of the present invention, the irradiation stepintroduces a total isolated vacancy concentration [V_(T)] which is atleast the greater of (a) 0.5 ppm and (b) 50% higher than the [N_(S) ⁰]concentration in the provided diamond material. The isolated vacancyconcentration [V_(T)] is given by the sum of [V⁰] and [V⁻], where [V⁰]is the concentration of isolated neutral vacancies, and [V⁻] is theconcentration of negatively charged isolated vacancies, both in ppm.Both [V⁰] and [V⁻] concentrations are determined from the GR1 and ND1absorption features of the absorption spectrum of the irradiated diamondin a manner described hereinafter. It is possible that said irradiationmight introduce vacancies in other forms, e.g. as pairs or in possibleisolated positive vacancies. The inventors did not observe any obviousfeatures in the material that could be associated with such defects, butdo not rule out this possibility. In certain embodiments according tothe invention the total isolated vacancy concentration [V_(T)] is morethan the greater of (a) 0.5 ppm and (b) 50% higher than the [N_(S) ⁰]concentration in the provided diamond material. For example the totalisolated vacancy concentration [V_(T)] may be at least 0.7 ppm, or atleast 0.9 ppm, or at least 1.0 ppm greater than the [N_(S) ⁰]concentration in the provided diamond material.

In general, the greater the irradiation dose, the greater the number ofisolated vacancies created. The number of isolated vacancies can dependnot only on the period of the irradiation dose but also on the numberand nature of defects in the provided CVD diamond material. Therefore inorder to calculate the desired dose of electronic radiation, theisolated vacancy production rate is also calculated for the givenirradiation conditions, as will be known to those skilled in the art.

Factors such as diamond temperature, beam energy, beam flux, and eventhe starting diamond's properties can affect the [V_(T)] produced for afixed experimental irradiation set-up and time. Irradiation is typicallycarried out with the sample mounted under ambient conditions ˜300 K withonly minimal temperature rise during the irradiation dose (e.g. lessthan 100 K). However, factors such as beam energy and beam flux can leadto sample heating. Optionally the sample is held as cold as possible(with even cryogenic cooling at 77 K being advantageous under somecircumstances) to enable high dose rates without compromisingtemperature control and thus minimize the irradiation time. This isadvantageous for commercial reasons. Calibration of the dose appliedagainst the vacancies produced for the particular provided diamond beingused in order to satisfy these limits on [V_(T)] concentrationintroduced will form part of the skilled person's responsibilitiesbefore carrying out the method of the present invention. Suchcalibration techniques are routine for the person skilled in the art.

A larger sample could be rotated and irradiated from two or more sidesin order to introduce vacancies through the whole thickness of thediamond material.

Optionally the provided diamond material may be annealed in thetemperature range 1400° C.-2500° C. prior to the first irradiation step.

Step (iii) of methods according to the invention comprises annealingirradiated diamond material at a temperature of at least 700° C. and atmost 900° C. for a period of at least 2 hours. The effect that thisannealing step has on the isolated vacancies in the irradiated diamondmaterial depends on whether, and how many, N_(s) ⁰ defects are presentin the irradiated diamond material. If there are N_(s) ⁰ defects in thediamond material, then initially annealing at 700° C. to 900° C. willform NV centers, each NV centre being the result of a N_(s) ⁰ defectjoining with a single isolated vacancy. In this case, when N_(s) ⁰defects are present, it is predominantly after the maximum number of NVcenters has formed that vacancy chains start to form. However, not allN_(s) ⁰ defects are converted to NV centers, this being thought be dueto the distribution of some of the N_(s) ⁰ defects. Once theconcentration of NV centers have saturated out, any isolated vacanciesthat have not been used up to form NV centers are available to combinewith each other to form vacancy chains. It is these vacancy chains thatare thought to give the orange color in the treated diamond materialaccording to the present invention. Therefore in methods according tothe present invention the irradiation of the provided CVD diamondmaterial such as to introduce isolated vacancies V into at least part ofthe provided CVD diamond material is such that the total concentrationof isolated vacancies [V_(T)] in the irradiated diamond material is atleast the greater of (a) 0.5 ppm and (b) 50% higher than the [N_(s) ⁰]concentration in ppm, so that there are sufficient excess isolatedvacancies over and above those that combine to form NV centers,available to join together to form vacancy chains.

If there are no N_(s) ⁰ defects in the provided CVD diamond material(and provided there is no other un-compensated element such as boronpresent) then when the annealing step (iii) of the methods according tothe invention is carried out the isolated vacancies formed during theirradiation step will start to group into vacancy chains immediately.

Any isolated vacancies remaining in the lattice typically result in aflatter UV-Visible (“UV-Vis”) optical absorption spectrum, whichtypically results in a more grey diamond material. Therefore for someembodiments, the concentration of isolated vacancies after the annealingprocess is complete is substantially reduced, and may be minimized. Forsome embodiments, after the annealing process the total concentration ofisolated vacancies is <0.3 ppm, optionally <0.2, optionally <0.1, oroptionally <0.05 ppm for a 0.5 carat (ct) round brilliant cut (rbc orRBC).

For many embodiments, the optimum anneal its one that will produce thehighest conversion of isolated vacancies to vacancy chains possible.Such embodiments may result in a vivid orange material, with a high C*value, typically C*>20.

Whilst not in the visible part of the spectra itself, an increase inabsorption centered at 250 nm is characteristic of the orange colorationand the saturation of the orange color scales with this feature.Therefore, a measure of the vacancy chain concentration is theabsorption at 250 nm. For some embodiments, after irradiation andannealing, for a 0.5 ct round brilliant cut diamond stone, theabsorption at 250 nm when measured at room temperature is >5 cm⁻¹,optionally >7 cm⁻¹, optionally >10 cm⁻¹ when the spectra has been scaledto 0 cm⁻¹ at 800 nm. Those skilled in the art will know that vacancyconcentrations will need to be altered for diamond stones with differentpath lengths to result in the stated the absorption coefficients.

An additional benefit to irradiating the CVD diamond material istypically the color of the material will be more stable tolow-temperature annealing and exposure to UV light (energy having anenergy of at least 5.5 eV) compared with untreated CVD diamond. Thisstabilization effect is discussed in GB application number 0911075.0 andU.S. application No. 61/220,663, both filed 26 Jun. 2009, and in GBapplication number 0917219.8 and U.S. application No. 61/247,735, bothfiled 1 Oct. 2009, the entire disclosures of which are incorporatedherein by reference.

In some embodiments according to the invention the provided diamondmaterial shows a measurable difference in at least one of its absorptioncharacteristics in first and second states, the first state being afterexposure to irradiation having an energy of at least 5.5 eV (typicallyUV light) and the second state being after thermal treatment at 798 K(525° C.), and after the irradiation and annealing steps of methodsaccording to the invention, the change in color saturation value C*between the diamond material in the first and second states is reducedby at least 0.5. This color stabilization may sometimes occur aftersimply irradiating. Optionally, after the irradiation and annealingsteps of methods according to the invention, the change in C* of thediamond material in the said first and second states is less than 1.

In general, the annealing step will be carried out after the irradiationstep is complete. However, it is also envisaged that there may be someoverlap of the irradiation and annealing processes, for example theannealing step may start before the irradiation step is complete, or thetwo processes may be carried out, and start and finish, at substantiallythe same time.

Typically annealing is carried out in an inert atmosphere e.g. an argonatmosphere or under vacuum. Annealing is typically carried out at <100mBar.

The present invention also provides the diamond material whenever madeby a method according to the first aspect of the invention.

A second aspect of the present invention provides a CVD diamond materialwhich when in the form of a 0.5 carat RBC is graded fancy orange.

The terminology fancy orange diamond material is defined as diamondswhich have a clear and distinct orange color (Diamond grading ABC TheManual, by Verena Pagel-Theisen, Rubin & Son, Belgium, 9^(th) Edition,2001, Page 67).

A third aspect of the present invention provides an orange CVD syntheticsingle crystal diamond having a hue angle in the range 69-90 for anequivalent 0.5 ct Round Brilliant Cut (RBC) diamond.

The perceived color of any particular diamond stone depends on the sizeand cut of the diamond. Therefore, where reference is given to the hueangle (which determines color), or to any color, it is usual in thefield to quote this in terms of a standard size, usually 0.5 carat, anda standard cut, usually round brilliant cut (often known as RBC or rbc)diamond stone. For any given diamond stone, be it larger or smaller than0.5 carat, or round brilliant cut or any other cut, models are availableto adjust the color to that for the standard size and cut. Therefore,the provided diamond material used in the method according to the firstaspect of the invention may have any size or cut, but color parameterswhere specified are adjusted to those for an equivalent material diamondstone of standard 0.5 carat size, and standard round brilliant cut forcomparison of quoted values.

Embodiments of the invention may have one or more of the following colorcharacteristics for an equivalent 0.5 ct Round Brilliant Cut (RBC)diamond stone.

TABLE 1 Characteristic Range Hue angle α 68°-90° optionally 69°-85°optionally 70°-80° Saturation C* 2-70 optionally 3-65 optionally 4-60Lightness L* >45 optionally >50 optionally >55

Material of this invention can be differentiated from as grown orangematerial, which has had no treatment, by features that are introducedduring the irradiation. These include small but measurable features inabsorption or PL when measured at 77 K or below. For example features at741 nm, 673 nm, 575 nm or 503 nm may be enhanced.

The color of the irradiated and annealed diamond can be quantified in awell established manner using “CIE L*a*b* Chromaticity Coordinates”. Theuse of CIE L*a*b* Chromaticity Coordinates in diamond is described in WO2004/022821, the entire disclosure of which is incorporated herein byreference. a* and b* are plotted as x and y axes of a graph and the hueangle is measured from the positive a* axis towards the positive b*axis. Thus a hue angle of greater than 90° and less than 180° lies inthe upper left quadrant of the a*b* graph. In this scheme for describingcolor L* is the lightness and a fourth coordinate C* is the saturation.

The perceived color of an object depends on the transmittance/absorbancespectrum of the object, the spectral power distribution of theillumination source and the response curves of the observer's eyes. TheCIE L*a*b* chromaticity coordinates (and therefore hue angles) quotedherein have been derived in the way described below. Using a standardD65 illumination spectrum and standard (red, green and blue) responsecurves of the eye (G. Wyszecki and W. S. Stiles, John Wiley, NewYork-London-Sydney, 1967) CIE L*a*b* chromaticity coordinates of aparallel-sided plate of diamond have been derived from its transmittancespectrum using the relationships below, between 350 nm and 800 nm with adata interval of 1 nm:

S_(λ)=transmittance at wavelength λ

L_(λ)=spectral power distribution of the illumination

x_(λ)=red response function of the eye

y_(λ)=green response function of the eye

z_(λ)=blue response function of the eye

X=Σ _(λ) [S ₈₀ x _(λ) L _(λ) ]/Y ₀

Y=Σ ₈₀ [S _(λ) y _(λ) L _(λ) ]/Y ₀

Z=Σ _(λ) [S _(λ) z _(λ) L _(λ) ]/Y ₀

Where Y₀=Σ_(λ)y_(λ)L_(λ)

L* = 116 (Y/Y₀)^(1/3) − 16 = Lightness (for Y/Y₀ > 0.008856) a* =500[(X/X₀)^(1/3) − (Y/Y₀)^(1/3)] (for X/X₀ > 0.008856, Y/Y₀ > 0.008856)b* = 200[(Y/Y₀)^(1/3) − (Z/Z₀)^(1/3)] (for Z/Z₀ > 0.008856) C* = (a*² +b*²)^(1/2) = saturation h_(ab) = arctan (b*/a*) = hue angle

Modified versions of these equations must be used outside the limits ofY/Y₀, X/X₀ and Z/Z₀. The modified versions are given in a technicalreport prepared by the Commission Internationale de L'Eclairage(Colorimetry (1986)).

It is standard to plot a* and b* coordinates on a graph with a*corresponding to the x axis and b* corresponding to the y axis. Positivea* and b* values correspond respectively to red and yellow components tothe hue. Negative a* and b* values correspond respectively to green andblue components. The positive quadrant of the graph then covers huesranging from yellow through orange to red, with saturations (C*) givenby the distance from the origin.

It is possible to predict how the a*b* coordinates of diamond with agiven absorption coefficient spectrum will change as the optical pathlength is varied. In order to do this, the reflection loss must first besubtracted from the measured absorbance spectrum. The absorbance is thenscaled to allow for a different path length and then the reflection lossis added back on. The absorbance spectrum can then be converted to atransmittance spectrum which is used to derive the CIE L*a*b*coordinates for the new thickness. In this way the dependence of thehue, saturation and lightness on optical path length can be modeled togive an understanding of how the color of diamond with given absorptionproperties per unit thickness will depend on the optical path length.

L*, the lightness, forms the third dimension of the CIE L*a*b* colorspace. It is important to understand the way in which the lightness andsaturation vary as the optical path length is changed for diamond withparticular optical absorption properties. The method described in thepreceding paragraph can also be used to predict how the L*C* coordinatesof diamond with a given absorption coefficient spectrum depend on theoptical path length.

The C* (saturation) numbers can be divided into saturation ranges of 10C* units and assigned descriptive terms as below.

 0-10 weak 10-20 weak-moderate 20-30 moderate 30-40 moderate-strong40-50 strong 50-60 strong-very strong 60-70 very strong  70-80+ veryvery strong

Similarly the L* numbers can be divided up into lightness ranges asfollows:

 5-15 very very dark 15-25 very dark 25-35 dark 35-45 medium/dark 45-55medium 55-65 light/medium 65-75 light 75-85 very light 85-95 very verylight

There are four basic color tones defined by the following combinationsof lightness and saturation:

Bright: Light and high saturation, Pale: Light and low saturation,

Deep: High saturation and dark, Dull: Low saturation and dark.

The stated hue angle, and a*, b*, C* and L* values provide aquantitative measure the quality and color of synthetic CVD diamondmaterial of the present invention. These color properties may beadvantageous because they give the diamond an orange color and can beused for ornamental purposes such as gemstones for jewelry, or for useas colored filters or similar.

For all samples used in this specification absorption peak heightsquoted in this specification are measured using a UV/visible absorptionspectrum of the synthetic CVD diamond material taken at roomtemperature.

All room temperature absorption spectra mentioned herein were collectedusing a Perkin Elmer Lambda-19 spectrometer. A reflection loss spectrumwas created using tabulated refractive index data and standardexpressions for the reflection loss for a parallel-sided plate. Therefractive index was determined according to Peter's equation [Z. Phys.,15 (1923), 358-368)] and subsequent reflection loss derived using thestandard Fresnel equation. The reflection loss spectrum was subtractedfrom the measured absorbance data and an absorption coefficient spectrumfor the sample is created from the resulting spectrum. Absorptioncoefficient data were shifted so that absorption coefficient was zero at800 nm.

Concentrations in ppm given in the present specification for thedifferent defects, [NV^(+/−)] and [V^(0/−)], may be calculated in aknown standard manner by integrating the area of peaks from theabsorption spectrum of the diamond usually collected at liquid nitrogentemperatures and using published coefficients for comparison tocalculate concentration. For concentrations of NV centers and isolatedvacancies, the spectra are advantageously obtained at 77K, using liquidnitrogen to cool the samples, since at that temperature sharp peaks at˜741 nm and ˜394 nm attributable to V⁰ and V⁻ and at 575 nm and 637 nmare seen attributable to NV⁰ and NV⁻ defects respectively. Thecoefficients that are used for the calculations of concentrations of NVcenters and isolated vacancies in the present specification are thoseset out by G. Davies in Physica B, 273-274 (1999), 15-23, as detailed inTable 2 below.

TABLE 2 Defect [label] Calibration (meV cm⁻¹) V⁻ [ND1] A_(ND1) = (4.8 ±0.2) × 10⁻¹⁶[V⁻] V⁰ [GR1] A_(GR1) = (1.2 ± 0.3) × 10⁻¹⁶[V⁰] NV⁻ A_(NV) ⁻= (1.4 ± 0.35) × 10⁻¹⁶[NV⁻] NV⁰ A_(NV) ⁰ = (1.4 ± 0.35) × 10⁻¹⁶[NV⁰]

In Table 2, “A” is the integrated absorption (meV cm⁻¹) in the zerophonon line of the transition, measured at 77 K, with the absorptioncoefficient in cm⁻¹ and the photon energy in meV. The concentration isin cm⁻³.

The provided CVD diamond material used in the method according to thepresent invention, and also the irradiated CVD diamond materialresulting from the method of the present invention may, or may not, formpart of a larger piece of diamond material. For example part only of thelarger piece of diamond material may be irradiated, and/or part only ofthe larger piece of diamond material may have the defined absorptioncharacteristics. As would be apparent to the person skilled in the artmultiple layers could also be irradiated and/or have the requiredabsorption characteristics, so that the provided CVD diamond materialused in the method according to the invention may form part, e.g. one ormultiple layers of a larger piece of diamond material. It is well knownthat the depth of penetration of irradiation is dependent on the energyof the irradiation. So in certain embodiments an irradiation energy isselected such that the irradiation penetrates only part of the depth ofa CVD diamond material. This means that isolated vacancies would beintroduced only in the penetrated part of the irradiated CVD diamondmaterial, and hence that penetrated part of the CVD diamond materialwould be the “diamond material” used formed by the method of the presentinvention.

Where the provided CVD diamond material provides only part of a largerpiece of diamond material, as discussed above that provided CVD diamondmaterial alone may have the advantageous optical properties describedfor certain embodiments of the invention. Thus for example a top orembedded layer or layers of a large piece of CVD diamond material mayhave an orange coloration. Where any other non-orange layers aresubstantially colorless the color of the larger piece of diamondmaterial is dominated by the orange layer(s).

In some embodiments according to the invention at least 50% or at least60% or at least 70% or at least 80% or at least 90% or substantially thewhole diamond stone may have substantially the same color.

In other embodiments according to the invention of diamond stone maycomprise layers or pockets of diamond material of the same color

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings and examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, which has been referred to hereinbefore, is a flow chart whichshows routes for methods according to the invention for obtaining orangediamond material;

FIG. 2 are UV visible absorption spectra measured at room temperaturefor examples 1 and 2, post irradiation and anneal; and

FIG. 3 are UV Visible absorption spectra measured at 77 K for examples3, 5 and 6 post irradiation and anneal.

EXAMPLES

HPHT diamond substrates suitable for synthesizing single crystal CVDdiamond of the invention were laser sawn, lapped into substrates,polished to minimize subsurface defects such that the density of defectsis below 5×10³/mm², and generally is below 10²/mm. Polished HPHT plates3.6 mm×3.6 mm square by 500 μm thick, with all faces {100} having asurface roughness R_(Q) at this stage of less than 1 nm were mounted ona refractory metal disk, and introduced into a CVD diamond growingreactor.

Growth Stages

-   -   1) The CVD diamond reactor was pre-fitted with point of use        purifiers, reducing unintentional contaminant species in the        incoming gas stream to below 80 ppb.    -   2) An in situ oxygen plasma etch was performed using 50/40/3000        sccm (standard cubic centimeter per second) of O₂/Ar/H₂ and a        substrate temperature of 760° C.    -   3) This moved without interruption into a hydrogen etch with the        removal of the O₂ from the gas flow.    -   4) This moved into the growth process by the addition of the        carbon source (in this case CH₄) and dopant gases. For these        examples the CH₄ flowing at 165 sccm, nitrogen was present in        the process gas at different levels for the different samples,        provided from a calibrated source for example 100 ppb N₂ either        as Air in Ar or N₂ in H₂, and for some examples O₂ was also        present in the process gas as shown in Table 3.

TABLE 3 Nitrogen dopant present in Oxygen flow present in Example theprocess gas (ppm) the process gas (ppm) 1 and 2 0.7 0 3 1.8 9160 4-6 1.113657

-   -   5) On completion of the growth period, the substrate was removed        from the reactor and the CVD diamond layer removed from the        substrate by laser sawing and mechanical polishing techniques.    -   6) This produced a CVD sample which had typical dimensions        ˜3.1×5×5 mm.

This grown CVD diamond is the “provided diamond” that is irradiated bymethods of the present specification.

The examples were electron irradiated a 4.5 MeV electron beam at 50%scan width and 20 mA beam current using an electron beam source such asthat found at Isotron plc. Diamond samples to be irradiated are mountedin indium on a water cooled copper block to prevent the samples beingheated above 350 K. The samples were then annealed in an Elite tubefurnace (model THS 16/50/180-2416CG and 27160/T). Typically to make anorange diamond material a dose of 5.8×10¹⁸ e⁻/cm² (equivalent to 6 hoursirradiation with a 4.5 MeV electron beam at 50% scan width and 20 mAbeam current) followed by an 8 hour anneal at 800° C. was used.

Table 4 records the CVD growth chemistry, the [N_(s) ⁰] concentration inthe provided diamond material, the absorption coefficients at 350 nm and510 nm and the color, of the provided diamond material, the irradiationdose, the vacancy concentration post irradiation, the annealing time andtemperature, the color of the diamond material post irradiation andanneal, the color characteristics, the [NV]. [V⁰] and [V⁻]concentrations and the absorption at 250 nm related to vacancy chains,all post irradiation and anneal. The results table 4 includes not onlyexamples falling within the scope of the present invention, but also anumber of comparative examples. For example, if the irradiation dose isnot high enough, the number of isolated vacancies available to combineto form chains upon annealing, irrespective of the length of the annealwill not be large enough to form a significant concentration of vacancychains; this is the case for comparative examples 2 and 6, which falloutside the scope of the present invention as the concentration ofisolated vacancies incorporated during the irradiation step is less thanthe greater of (a) 0.5 ppm and (b) 0.5 ppm more than the [N_(s) ⁰]concentration, and the absorption at 250 nm in the treated sample is <5cm⁻¹. This is also illustrated with reference to FIG. 2 which is a roomtemperature UV visible absorption spectrum for examples 1 and 2, postirradiation and anneal. This figure shows strong absorption at 250 nmfor example 1, indicating the presence of vacancy chains, whereas inexample 2 the absorption in the 250 nm range is less than 5 cm⁻¹,showing a low concentration of vacancy chains has been formed. Similarlywe have found that if the annealing time is not long enough then totalconcentration of isolated vacancies remaining in the treated sampleis >0.3 ppm; this is the case in comparative example 5, which isannealed for only 1 hour and results in a grey colored diamond materialas compared with the orange color achieved with example 4, which is anidentical diamond material sample to that of example 5 in terms ofcomposition and irradiation, but is annealed for a longer time.

FIG. 3 which shows UV visible spectra taken at 77 K post irradiation andanneal and illustrates for example 3 strong absorption at 250 nm and nopeak at 741 nm or 394 nm remaining, showing that substantially all ofthe isolated vacancies have been annealed out. FIG. 3 also illustrateswhy comparative example 5 (which has been annealed for an insufficienttime) appears grey post irradiation and anneal since there are peaks at741 nm and 394 nm indicating the presence of isolated vacancies and alsoat 575 nm and 637 nm showing the presence of NV centers. Similarly FIG.3 illustrates why comparative example 6 (which has been subjected toinsufficient irradiation dose) appears pale pink, since there are peaksat 575 nm and 637 nm, showing the presence of NV centers, a smallconcentration of isolated vacancies remaining, and weak absorption at250 nm indicating a low concentration of vacancy chains.

All of the orange diamond samples according to the invention (examples1, 3 and 4, show strong absorption at around 250 nm. This absorption isbelieved to be due to the presence of vacancy chains. For example, themeasured absorption at 250 nm is >5 cm⁻¹ for both samples 1 and 3,whereas for comparative sample 2 it is <5 cm⁻¹.

As noted above an additional benefit of irradiating the CVD diamondmaterial is that typically the color of the material will be more stableto low temperature annealing and exposure to UV light compared tountreated CVD diamond. We found that upon heating example 1 the changein C* between the two states was <1 which illustrates this benefit.

TABLE 4 N_(s) ⁰ conc. Color in the in provided Vacancy Anneal timeprovided diamond pre- concentration and CVD CVD irradiation Irradiationpost temperature Example Growth diamond Abs Abs (Color grade if doseirradiation (hours) Number chemistry (ppm) at 350 nm at 510 nm 0.5 ctRBC) (e/cm²) (ppm) (° C.) 1  Traditional 0.1 1.09 0.45 Colorless 5.8 ×10¹⁸ V⁰ = 1.41 8 hrs at CVD V⁻ = 0.03 800° C. growth process 2*Traditional 0.1 1.09 0.45 Colorless 2.6 × 10¹⁷ V⁰ = 0.17 8 hrs at CVD V⁻= 0.033 800° C. growth process 3  Added 0.6 1.85 0.60 Pale yellow 3.9 ×10¹⁸ V⁰ = 1.95 8 hrs at oxygen V⁻ = 0.23 800° C. CVD growth process 4 Added 0.35 1.42 0.63 Pale yellow   2 × 10¹⁸ V⁰ = 1.1 8 hrs at oxygen V⁻= 0.3 800° C. CVD growth process 5* Added 0.35 1.42 0.63 Pale yellow   2× 10¹⁸ V⁰ = 1.1 1 hr at 800° C. oxygen V⁻ = 0.3 CVD growth process 6*Added 0.35 1.42 0.63 Pale yellow 2.6 × 10¹⁷ V⁰ = 0.06 8 hrs at oxygen V⁻= 0.12 800° C. CVD growth process Absorption at 250 nm [V⁰] and [V⁰]related to Observed Color [NV] concentration vacancy color ofcharacteristics concentration (ppm) chains at RT diamond after L* (ppm)Post Post Example irradiation and C* Post irradiation irradiation andirradiation and Number annealing α and anneal anneal anneal 1  Brightvivid L* = 63.1 NV⁰ = 0.035 V⁰ = <0.003 22.01 orange C* = 52.6 NV⁻ =0.0004 V⁻ = <0.003 α = 78.2° 2* Dull pinkish L* = 80.2 NV⁰ = 0.031 V⁰ =0.046 2.04 brown C* = 5.05 NV⁻ = 0.0086 V⁻ = 0.0086 α = 67.7° 3  Brightvivid L* = 59.1 NV⁰ = 0.19 V⁰ = <0.003 13.74 orange C* = 34.4 NV⁻ =0.038 V⁻ = <0.003 α = 70.7° 4  Orange-pink L* = 70.5 NV⁰ = 0.092 V⁰ =0.21 7.8 C* = 17.08 NV⁻ = 0.019 V⁻ = 0.034 α = 71.7° 5* Grey L* = 69.3NV⁰ = 0.055 V⁰ = 0.45 8.71 C* = 10.8 NV⁻ = 0.019 V⁻ = 0.053 α = 83.3° 6*Pale pink L* = 87.4 NV⁰ = 0.078 V⁰ = 0.041 3.01 C* = 4.26 NV⁻ = 0.14 V⁻= 0.044 α = 44.93°

1. A method of making fancy orange synthetic CVD diamond material, themethod comprising: (i) irradiating a single crystal diamond materialthat has been grown by CVD and has a [N_(s) ⁰] concentration less than 5ppm to introduce isolated vacancies V into at least part of the CVDdiamond material, the total concentration of isolated vacancies [V_(T)]in the irradiated diamond material being at least the greater of (a) 0.5ppm and (b) 50% higher than the [N_(s) ⁰] concentration in ppm in thesingle crystal diamond material, and (ii) annealing the irradiateddiamond material to form vacancy chains from at least some of theintroduced isolated vacancies.
 2. A method according to claim 1, whereinthe annealing is carried out at a temperature of at least 700° C. and atmost 900° C.
 3. A method according to claim 1, wherein the annealing iscarried out for a period of at least 2 hours.
 4. A method according toclaim 1, wherein the annealing steps reduce the concentration ofisolated vacancies in the irradiated diamond material, whereby theconcentration of isolated vacancies in the irradiated and annealeddiamond material is <0.3 ppm.
 5. A method according to claim 1, whereinthe absorption coefficients at 350 nm and 510 nm for the diamondmaterial prior to irradiation are less than 3 cm⁻¹ and 1 cm⁻¹respectively.
 6. A method according to claim 1, wherein the atomic boronconcentration [B] in the diamond material is less than 5×10¹⁵ cm⁻¹.
 7. Amethod according to claim 1, wherein uncompensated boron is present inthe diamond material in a concentration of >5×10¹⁵ cm⁻³, and theirradiation step (ii) introduces sufficient isolated vacancies into thediamond material so that total concentration of isolated vacancies[V_(T)] in the irradiated diamond material, after isolated vacancieshave been used to compensate the boron, is at least the greater of (a)0.5 ppm and (b) 50% higher than the [N_(s) ⁰] concentration in ppm inthe diamond material prior to its irradiation.
 8. A method according toclaim 1, wherein the diamond material is irradiated from two or moresides.
 9. A method according to claim 1, wherein at least 50% of the CVDdiamond has been formed from a single growth sector.
 10. A methodaccording to claim 1, wherein, after the irradiation and annealing steps(ii) and (iii), the absorption in the 250 nm region of the irradiatedand annealed diamond material, when measured at room temperature, isgreater than 5 cm⁻¹
 11. A method according to claim 1, wherein thediamond material prior to irradiation according to step (i) of themethod shows a measurable difference in at least one of its absorptioncharacteristics in first and second states, the first state being afterexposure to irradiation having an energy of at least 5.5 eV and thesecond state being after thermal treatment at 798K (525° C.), andwherein after the irradiation and annealing steps of the method thechange in color saturation value C* between the diamond material in thesaid first and second states is reduced by at least 0.5 compared to thechange in color saturation value C* between the diamond material in thesaid first and second states for the diamond material prior to itsirradiation.
 12. A method according to claim 1, wherein after theirradiation and annealing steps of the method, the change in colorsaturation C* of the diamond material in first and second states is lessthan 1, the first state being after exposure to irradiation having anenergy of at least 5.5 eV and the second state being after thermaltreatment at 798 K (525° C.).
 13. A method according to claim 1, whereinthe diamond material is annealed in the temperature range 1400° C.-2500°C. prior to the irradiation step.
 14. A method according to claim 1,wherein the annealing step (ii) of method claim 1 is carried out afterirradiation step (i) of method claim 1 is complete.
 15. CVD diamondmaterial when made by a method according to claim
 1. 16. CVD diamondmaterial which for an equivalent 0.5 carat Round Brilliant Cut (RBC) isgraded fancy orange.
 17. CVD synthetic single crystal diamond materialhaving the following color characteristics measured for an equivalent0.5 ct Round Brilliant Cut (RBC) diamond: Characteristic Range Hue angleα 68°-90° Saturation C*  2-70 Lightness L* >45


18. CVD diamond material according to claim 16, wherein theconcentration of isolated vacancies is <0.3 ppm.
 19. CVD diamondmaterial according to claim 16, wherein for an equivalent 0.5 ct roundbrilliant cut diamond stone, the absorption in the 250 nm region whenmeasured at room temperature is >5 cm⁻¹.
 20. CVD diamond materialaccording to claim 16, wherein the change in saturation C* of thediamond material in first and second states is less than 1, the firststate being after exposure to irradiation having an energy of at least5.5 eV and the second state being after thermal treatment at 798 K (525°C.).
 21. Jewelry comprising diamond material according to claim 16 and asetting for the diamond material.
 22. A round brilliant cut diamondgemstone comprising diamond material according to claim 16.